Traditional IC sockets are generally constructed of an injection molded plastic insulator housing that includes stamped and formed copper alloy contact members stitched or inserted into recesses. The assembled IC socket is then generally processed through a reflow oven to attach solder balls to the contact members. During final assembly the contact pads on the printed circuit board (“PCB”) are printed with solder paste or flux and the solder balls on the IC socket are placed in registration with the contact pads. The assembly is then reflowed and the solder balls essentially weld the IC socket to the PCB.
During use, an IC socket receives an IC device, such as a packaged integrated circuit. The contact members electrically couple the terminals on the IC device with the corresponding terminal on the PCB. The terminals on the IC device are typically held against the contact members by applying a load, which is expected to maintain intimate contact and reliable circuit connection throughout the life of the system without a permanent connection. As a result, the IC device can be removed or replaced without the need for reflowing solder connections.
These types of IC sockets and interconnects have been produced in high volume for many years. As IC devices advance to next generation architectures traditional IC sockets have reached mechanical and electrical limitations that require alternate methods.
As processors and systems have evolved, several factors have impacted the design of traditional IC sockets. Increased terminal counts, reductions in the distance between the contacts known as terminal pitch, and signal integrity have been main drivers that impact the socket and contact design. As terminal counts go up, the IC package essentially gets larger due to the additional space needed for the terminals. As the package grows larger, costs go up and the relative flatness of the package and corresponding PCB require compliance between the contact and the terminal pad to accommodate the topography differences and maintain reliable connection.
Package producers tend to drive the terminal pitch smaller so they can reduce the size of the package as well as the flatness effects. As the terminal pitch reduces, the available area to place a contact is also reduced, which limits the space available to locate a spring or contact member which can deflect without touching a neighbor. In order to maximize the length of the spring so that it can deflect the proper amount without damage, the thickness of the insulating walls within the plastic housing is reduced which increases the difficulty of molding as well as the latent stress in the molded housing, resulting in warpage during the heat applied during solder reflow. For mechanical reasons, the contact designs desire to have a long contact that has the proper spring properties. Long contact members tend to reduce the electrical performance of the connection by creating a parasitic effect that impacts the signal as it travels through the contact. Other effects such as contact resistance impact the self-heating effects as current passes through power delivering contacts, and the small space between contacts can cause distortion as a nearby contact influences the neighbor which is known as cross talk. Traditional socket methods are able to meet the mechanical compliance requirements of today's needs, but they have reached an electrical performance limit.
Traditional test sockets are manufactured from bulk plastic material that is machined to provide device location features as well as positions for the electrical contacts that can be stamped and formed, blanked, wire electro-discharge machining processed, or constructed from conductive elastomer, coil spring probes, or several variations. The predominant contact type used in test sockets is the spring probe, which basically consists of two or more metal members that engage each other to create the electrical path biased by a coil spring that provides normal and return force.
Next generation systems will operate above 5 GHz and beyond and the existing interconnects will not achieve acceptable performance levels without significant revision. A major issue with the use of spring probes in test sockets is the electrical performance is degraded by the coil spring which is an inductor, as well as the potential capacitance of the metal members and the relatively high contact resistance due to the various sliding connection point.